Binders for solid particles based on hydrolyzates of silicic acid esters or silica gels are known to be useful as binders in the paint industry. The pigmentation of such binders with zinc dust, for example, produces paints which are known in the art as zinc dust paints or zinc-rich paints; they are used for providing metals, especially iron and steel, with corrosion-resistant coatings. The cured coatings are characterized by a high degree of surface hardness, and therefore excellent abrasion resistance, as well as by good solvent and temperature resistance and electrical conductivity.
The development of zinc dust paints based on silica gels began with two-component systems, where, in the simplest case, one component is a hydrolyzate of ethyl silicate in alcoholic solution as a binder, and the other is zinc dust. The two components are not mixed until just prior to application of the composition to the substrates, since the coating composition solidifies within a few hours after the zinc dust is added (See U.S. Pat. No. 3,056,684).
The subsequent developments made use of the effect of various additives upon the workability and quality of the paints, for examples by the addition of polyvinylbutyral (U.S. Pat. No. 3,392,130), boric acid esters (U.S. Pat. No. 3,392,036), alkyl titanates (U.S. Pat. Nos. 3,442,824 and 3,546,155) or trialkyl phosphates (German Patent No. 2,000,199). To raise the flash point of the binders, ethyl silicate was transesterified with higher alcohols (U.S. Pat. No. 3,730,743). However, two-component zinc dust paints have a number of disadvantages as regards their use. Because of the necessity of storing the binder and zinc dust separately, there are disadvantages with respect to stockkeeping. Once made up, the mixtures must be processed immediately and completely within a few hours. Moreover, the aging of the hydrolyzate during storage results in changes of important performance parameters, such as processing properties (for instance, viscosity) and reactivity (effect of curing rate), so that considerable problems arise when zinc dust paints are applied which contain binders of different degrees of aging.
Because of the above-mentioned disadvantages of two-component systems, the so-called one-component systems were developed during the ensuing years. These improved systems were intended to bring about improvements, especially with respect to the stability of the mixture of binder, pigments, and fillers in the paint container. Because of the high density of the zinc dust and the low viscosity of the alkyl silicate, the zinc dust settled on the bottom during storage in closed containers in such a way that it was no longer possible or possible only with great difficulty to redisperse the precipitate by stirring. The use of specific rheological additives, such as Bentones.RTM. or pyrogenic silicic acid, made it possible to produce single-component zinc dust paints which can be used without difficulty even after several months of storage. These formulations contain unhydrolyzed silicic acid esters and react only under the action of atmospheric moisture, by hydrolysis and condensation, to give an increasingly crosslinked SiO.sub.2 -gel (See U.S. Pat. No. 3,859,101 and German Patent No. 2,654,036). The hydrolysis and condensation process is catalyzed by alkaline substances such as amines or alcoholates.
Because of the use of alcohols, ketones, ethers and alkyl esters of lower carboxylic acids, or mixtures thereof as solvents, the known binders have a flash point which is often less than 21.degree. C. Since small quantities of alcohol are released due to hydrolysis when the zinc dust paint is applied, it is difficult for binders based on ethyl silicate to attain a flash point higher than 21.degree. C. This is also true if solvents or solvent mixtures are employed which originally have a flash point substantially higher than 21.degree. C. When xylene is used as the solvent, the flash point of the binder is 23.degree. C. (See European Patent No. 0,039,008). When propyl or butyl silicates are used, the systems obtained thereby also have relatively high flash points. In both instances, however, because of the relatively slow evaporation of the solvent (xylene or the alcohols formed) and because of the modified hydrolysis characteristics of the esterified silicic acid groups, the initial curing of the paint is delayed.